Method of constructing a magnetic core

ABSTRACT

A method of constructing a magnetic core for electrical inductive apparatus, such as transformers and reactors, having stepped-lap joints between adjoining leg and yoke portions thereof. The method provides a continuous stepped-lap joint which extends around each corner of the magnetic core, while incrementally clipping only one end of each leg and yoke lamination, and the laminations are assembled without any portion thereof protruding past the major outline of the magnetic core configuration.

United States Patent 11 1 1111 3,918,153

Burkhardt et al. 1451 Nov. 11, 1975 METHOD OF CONSTRUCTING A 2,898,565 8/l959 Fox et al 336/217 M E CORE 3,540,120 ll/197O DeLaurentis et a1. 29/609 [75] Inventors: Charles E. Burkhardt Sharon' Primary E.\'ammer.l0hn F. Campbell Belvm Pulaskl both of Assistant Examiner-Carl E. Hall [73] Assignee: Westinghouse Electric Corporation, Attorney, g or Lackey Pittsburgh, Pa. 22 Filed: Mar. 22, 1971 [57] ABSTRACT A method of constructing a magnetic core for electri- [211 App! 126632 cal inductive apparatus, such as transformers and reactors, having stepped-lap joints between adjoining leg [52] US. Cl. 29/609; 336/217; 336/234 and yoke portions thereof. The method provides a [51] Int. Cl. HOlF 41/02 contin ous s epped-lap joint which extends aro nd [58] Field of Search 29/602, 609; 336/216, 217, each corner of the magnetic core, while incrementally 336/234 clipping only one end of each leg and yoke lamination, and the laminations are assembled without any portion [56] References Cited thereof protruding past the major outline of the mag- UNITED STATES PATENTS, netic core configuration. 2,628,273 2/1953 Somerville 29/609 X 7 Claims, 16 Drawing Figures 96 EDI y/// METHOD OF CONSTRUCTINGA MAGNETIC CORE BACKGROUND OF THE INVENTION 1. Field of the Invention:

The invention relates in general to magnetic cores for electrical inductive apparatus, and more specifically to methods of constructing magnetic cores of the stacked type for such apparatus.

2. Description of the Prior Art:

The stepped-lap type of joint in a magnetic core structure, i.e., where the joints between the leg and yoke portions of the core, ineach layer of the laminations, are incrementally offset from similarly located joints in adjacent layers, in a predetermined stepped or progressive pattern, substantially improves the performance of a core, compared withthe butt-lap type joint. Examples of the stepped-lap construction are shown in U.S. Pat. Nos. 3,153,215; 3,477,053 and 3,540,120, and the butt-lap construction is shown in U.S. Pat. No. 2,300,964, all of which are assigned to the same assignee as the present application. Y

It has been found that the performance of the stepped-lap magnetic core structure can be optimized by increasing the number of steps in the stepped-lap pattern before the pattern repeats, and by increasing the amount of overlap between adjacent joints. The larger overlap is also desirable in larger magnetic cores, as it increases the mechanical strength of the core and makes stacking of long laminations less critical from a tolerance viewpoint. Increasing the number of joints per basic pattern, and the amount of overlap between adjacent joints, however, increases the void volume generated at the corners of the core opening or window, which is undesirable" due to the natural "tendency of the magnetic flux to hug thesein'ner corners of the magnetic core. Increasing the void volume at these inner corners causes the magnetic flux to move outwardly, away from these corners, resulting in flux crowding, a longer mean flux-path, "and'more cross grain flux, increasing the 'losses o'f the magnetic core. Thus, the corner void volume generated places severe restrictions on the number of steps which may be used in the basic stepped pattern, and the amount of overlap.

U.S. Pat. No. 3,477,053 discloses a structure which reduces the void volume at the window corners by transferring a portion of the void volume to the 'outer comers of the magnetic core, where it has less adverse affect on the losses of the magnetic core, but the disclosed construction requires that both' ends of certain laminations be clipped, in order to prevent the ends of certain of the laminations from protruding past the major outline configuration of the magnetic core. In certain applications protruding ends may cause. no problem, but in other applications they may interfere with the end frames, or with form-fit tanks, and in those applications they must be clipped to conform to the major core outline. It would be desirable to be able to increase the number of steps in the pattern without increasing the void volume, while clipping only' one end of each of the laminations, and still not'have any of the laminations protrude beyond the major outline of the magnetic core. i Y

U.S. Pat. No. 3,540,120 discloses stepped-lapjoint constructions which generate no void volume. How ever, die cutting is required, which'is a disadvantage 2 where it is desirable to utilize existing shear equipment. Thus, it would be desirable to cut all of the laminations of the stepped-lap core with a shear, which cuts completely across the width dimension of the sheet to form the required leg and yoke laminations.

SUMMARY OF THE INVENTION Briefly, the present invention is a new and improved method of constructing magnetic cores with steppedlap joints between the adjoining leg and yoke portions, which method substantially doubles the number of joints in the stepped pattern without any protrusions beyond the major core outline configuration, while clipping only one end of the laminations, and without generating any more void volume at the corners of the window or opening in the magnetic core than prior art patterns having approximately half the number of steps. Further, the resulting basic stepped pattern is located on both sides .of each corner of the magnetic core, with this construction being accomplished even though the ends of groups of laminations are incrementally clipped and aligned against a plane surface.

Briefly, these results are obtained by cutting trapezoidal shaped leg and yoke laminations from a strip of magnetic material, with the leg laminations all being of like dimension on the obtuse angle side of the trapezoid, and the yoke laminations all being of like dimension on the obtuse side of the trapezoid. The leg and yoke laminations are incrementally clipped on one end only to provide a predetermined number of leg and yoke laminations having different maximum length dimensions on the acute angle side of the trapezoid. These different length leg and different length yoke laminations are stacked in different groups with the incrementally clipped ends against" a plane surface, to step or space the diagonally cut edges of the laminations of the stack in a predetermined progressive arrangement. Two groups of leg laminations and two groups of yoke laminations are assembled, which groups are all similarly oriented with the longest lamination on. one side of the group and the shortest lamina-' tion. on the other side of the group, to provide a first assembly. Then, two groups of leg laminations and two groups of yoke laminations are assembled to form a second assembly, and the second assembly is superposed on the first assembly, with the second assembly being similar to the first except in rotational symmetry therewith about an axis of the first assembly which is parallel to the major planes of the laminations therein. In other words, the longest laminations of the second assembly are disposed against the longest laminations of the first assembly. The unclipped end of the longest lamination of each group is placed to start at an outer geometrical corner of the magnetic core configuration. Thus, the laminations do not protrude past the major core outline configuration. The void volume at the inner corners of the core iseffectively reduced because it is shifted back and forth from assembly to assembly, between the leg and yoke portions of the core. The number of joints in the stepped pattern is substantially doubled, without increasing the effective void volume, compared with a magnetic core having one half the number of steps in the basic stepped-lap pattern, and the laminations may all be formed with a 'shear,-as each lamination is cut with a single straight cut, completely across the width dimension of the strip from which it is formed.

BRIEF DESCRIPTION OF THE DRAWINGS The invention may be better understood, and further advantages and uses thereof more readily apparent, when considered in view of the following detailed description of exemplary embodiments, taken with the accompanying drawings, in which:

FIG. 1 illustrates a strip of magnetic, metallic material, with the pattern shown thereon for cutting leg and yoke laminations according to the new andimproved method of constructing a magnetic core;

FIGS. 2A-2E illustrate trapezoidal shaped yoke laminations cut from the strip of magnetic, metallic material shown in FIG. 1, with the incremental clipping pattern indicated thereon;

FIGS. 3A-3E illustrate trapezoidalshaped leg laminations cut from the strip of magnetic, metallic material shown in FIG. 1, with the incremental clipping pattern indicated thereon;

FIG. 4 illustrates the stacking and the start of the assembly of two groups of leg laminations with two groups of yoke laminations, to provide a first basic assembly of groups;

FIG. 5 is a plan view of the first basic assembly after the assemblyof the groups of leg and yoke laminations has been completed;

FIG. 6 is a plan view of the second basic assembly, which is similar to the first basic assembly, except in 180 rotational symmetry therewith about an axis parallel with the planes of the major surfaces of the laminations; 7

FIG. 7 is a plan view illustrating the second basic assembly superposed on the first basic assembly, to form a composite assembly; and

FIG. 8 is a cross sectional view of the basic steppedlap pattern of the composite assembly shown'in FIG. 7, taken in the direction of arrows VIII-VII'I.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to the drawings, the figures illustrate the steps of a new and improved method of forming a magnetic core 9, shown in a plan view in FIG. 7, having first and second leg portions 22 and 24, respectively, disposed in spaced parallel relation, the adjacent ends of which are joined by first and second yoke portions 26 and 28, respectively. The assembled leg and yoke portions define a substantially rectangular window or opening 29, with the opening having corners'92, 94, 96 and 98. The magnetic core 9 also has a substantially rectangular outer configuration, having four outer corners referenced 100, 102, 104 and 106. While portions 22 and 24 are referenced leg portions, and portions 26 and 28 are referenced yoke portions, it will be understood that their functions may be interchanged, de-

pending upon the specific type of magnetic core desired. Further, the magnetic core 9 may function as one-half of a magnetic core of the shell-formed type, wherein a similar structure would be placed adjacent thereto, with the windings of the electrical apparatus proceeding through the openings in both of the magnetic core structures.

The first step of the new and improved method of constructing magnetic core 9 is to provide a strip 10 of metallic, magnetic material, as shown in FIG. 1. Strip 10, which is preferably formed of grain oriented material, such as silicon steel having apreferred direction of magnetic orientation which is parallel with the sides 17 and 19 of the strip, has a predetermined width dimento show where spaced, straight cuts 11, 13 and 15 may be made to provide a plurality of trapezoidal shaped yoke laminations for the magneticcore 9, such as yoke laminations 14 and 16, and a plurality of trapezoidal shaped leg laminations for the magentic core 9, such as leg laminations 18 and 20. The cuts 11, 13 and 15 extend'across the complete width dimension of strip 12, at an angle of substantially 45 relative to the sides or edges 17 and 19 of the strip 10. Thus, the leg and yoke laminations may be assembled with their diagonally cut ends butting together, to provide a joint which presents the least reluctance to magnetic flux, for strip which has a preferred direction of magnetic orientation parallel with the edges 17 and 19 of the strip 10. Since the cut lines 11, 13 and 15 are straight, and extend completely across the width dimension 12 of the strip 10, the cuts may be conveniently made with ashear.

Magnetic core 9 shown in FIG. 7 has stepped-lap joints between the various leg and yoke portions, in

which similarly located joints are offset from one another, from layer to layer, in a predetermined stepped pattern, with the joints progressing at least three steps in one direction before repeating or changing direction. The stepped pattern is achieved by forming all of the yoke laminations to like predetermined dimensions, and all of the leg laminations to like predetermined dimensions, and then incrementally cutting one end of the leg and yoke laminations to form a plurality of different dimensioned leg and yoke laminations. The number of different yoke and leg laminations, and the increments used to cut the different maximum'lengths of these laminations, will depend upon the number of steps desired in the basic pattern, and the amount of overlap desired between adjacent joints. In the prior art, the maximum tolerable void volume generated at the inner corners of the core, i.e., at the window corners, limited the overlap dimension and the number of steps in the basic pattern. The present invention enables the number of steps in the basic pattern to be substantially doubled, without doubling the effective void volume at the inner corners of the core. Thus, if the same number of steps are desired, as in the prior art core, the effective void volume may be reduced, or given a predetermined maximumv effective void volume, the overlap and number of steps in the basic pattern may be both increased by following the teachings of the present invention, compared with the teachings of the prior art, without requiring die cutting of the laminations, and without clipping both ends of certain of the laminations to prevent them from extending beyond the sides of the magnetic core.

For purposes of example, it will be assumed that the pattern is to have nine steps in succession before repeating, four on each side of the comer of the magnetic core, and one at the corner. The overlap will depend upon the size of the magnetic core, with /8 inch overlap being common for small magnetic cores, and onefourth or even three-eighths inch common for the larger magnetic cores.

The nine step pattern requires five different length dimensions for both the yoke and leg laminations, when constructing the core according to the teachings of the invention, with FIGS. 2A, 2B, 2C, 2D and 2E indicating yoke laminations 30, 32, 34, 36 and 38, respectively, which were similarly dimensioned prior to the step of incrementally cutting one of their ends. The incremental cuts start a predetermined dimension-from one end and the cut is made perpendicularto the side of the lamination and it intersects the diagonally cut end of the lamination. For examplefla mination30 shown in FIG. 2A is out along line 40 a predetermined dimension from end 42, and, the small piece 44 is removed from the lamination. This reduces the overall length of lamination 30 on the acute angle side of the trapezoid to the dimension 41, while the dimension 43 on the obtuse angle side of the trapezoidal lamination has been unchanged. Lamination 32, shown in FIG. 2B, is cut along line 46, which is further from point 42 than cut 40 on lamination 30, by the selected increment. In, like man-- ner, lamination 34, 36 and 38 shown in FIGS. 2C, 2D and 2E, respectively. are incrementally cut along lines 48, 50 and 52, respectively. Thus, the maximum length dimensions of laminations 30, 32,34, 36 and 38 are decreased in predetermined increments. The lengths of the increments are purposely exaggerated in the drawings, relative to the overall sizes of the lamination, in order to make the incremental clipping of the laminations more readily apparent.

FIGS. 3A, 3B, 3C, 3D and 3E illustrate leg laminations 54, 56, 58, 60 and 62, respectively, which are incrementally cut along lines 64, 66, 68, 70 and 72, respectively. The maximum length dimensions of laminations 54, 56, 58, 60 and 62 are thus decreased in predetermined increments, reducing their maximum overall length, without affecting the shorter dimension of each lamination between the two diagonal cuts. In other words, the maximum length of lamination 54 shown in FIG. 3A is reduced to dimension 65, while dimension 67 remains unchanged by the incremental cutting.

The steps of incrementally cutting. one end of each leg and yoke lamination may be conveniently performed while shearing the lamination from the strip 10. The leading edge of strip may be clipped atthe same time as the associated lamination is being sheared from the strip 10.

The next step, shown in FIG. 4, is the stacking of the laminations into groups. The yoke laminations are stacked into groups, each of which includes at least one of the different incrementally cut yoke laminations. In like manner, the leg laminations are stacked into groups, with each group including at least one of the different incrementally cut leg laminations. The different laminations in each group are sequenced such that the lamination with the longest length dimension is on one side of the group, and the lamination with the shortest length dimension is on the opposite side of the group, with the length dimensions of the remaining laminations decreasing in steps between the longest and shortest laminations.

FIG. 4 illustrates first and second groups or'stacks 74 and 76, respectively, of leg laminations, and first and second groups 78 and 80, respectively, of yoke laminations. All of the groups shown in FIG. 4 are similarly stacked, with the longest lamination on top of the stack and the shortest on the bottom, as viewed in FIG. 4, with the clipped ends of the laminations in each group being on the same end of the group. The clipped ends of each group are all placed against a plane surface while superposing the edges of the laminations, to incrementally step or space the diagonally cut edges of the laminations at both ends thereof. Group 74 is thus stacked such that lamination 62 is on the bottom, followed in sequence by laminations 60, 58, 56, and 54. Group 76 is similarly stacked, with the reference nu- 6 merals of the laminations of this group including a prime mark'to distinguish them from the reference numerals of the laminations of group 74.

Group 78 is stacked with yoke lamination 38 on the bottom of the stack, followed in sequence by laminations 36, 34, 32 and 30. Group is similarly stacked. with the reference numerals of the laminations of group 80 including a prime mark to distinguish them from like laminations of group 78.

The next step, shown in FIGS. 4 and 5, is assembling the first and second groups 74 and 76, respectively, of leg laminations with the first and second groups 78 and 80, respectively, of yoke laminations. As hereinbefore pointed out, these groups of leg and yoke laminations are oriented with the longest lamination on top of each group. FIG. 4 illustrates the groups or stacks 74 and 76 of leg laminations disposed in the required spaced par allel relation, with groups 78 and 80 of yoke laminations shown just prior to their assembly with groups 74 and 76. Assembly of the groups requiresthat groups 78 and 80 be moved toward one another, as indicated by arrows 82 and 84, respectively, until the diagonally cut edges of the laminations of the groups 78 and 80 butt against the diagonally cut edges of groups 74 and 76, to provide a first basic assembly 90, shown in FIG. 5, having a plurality of layers of laminations, with stepped-lap joints formed at the junctions of the yoke and leg groups. The first basic assembly defines a substantially rectangular opening or window 29, dimensioned similar to the opening 29 in the magnetic core 9 shown in FIG. 7, with the opening 29 having corners 92', 94', 96 and 98'. The outer configuration of the first basic assembly 90 has corners 100, 102, 104 and 106.

The adjoining groups of leg and yoke laminations are placed relative to one another, such that the laminations in the top layer, as viewed in FIGS. 4 and 5, fit together with substantially no void volume at the inner corners. Further, the generated void volume at the corners of the opening 29 alternate between the leg and yoke portions from corner to corner of the window 29'. In other words, the groups of laminations are assembled such that the start of the diagonal cuts on the laminations of the top layer are substantially in contact with each other, eliminating any void volume in this layer of laminations. The void volume generated at corner 92 is in the leg portion of the core, the void volume generated at corner 94 is in the yoke portion, the void volume generated at corner- 98 is in the leg portion, and the void volume generated at corner 96 is in the yoke portion.

The next step of the method is providing a second basic assembly 110 of groups of leg and yoke laminations, as shown in FIG. 6. The second basic group 110 is similar to the first basic group 90 shown in FIG. 5, except the second basic group is in rotational symmetry with the first basic assembly 90 about an axis of basic assembly 90 which is parallel with the major planes of the laminations of basic assembly 90. In other words, if the first assembly 90 were to be rotated 180 about axis 112 disposed through the groups 74 and 76 of leg laminations, as illustrated by arrow 114, or if it were to be rotated 180 about axis 116 disposed through groups 78 and 80 of yoke laminations, as illustrated by arrow 118, the resulting assembly would be that shown in FIG. 6. However, in providing the second assembly 1 10, it is not necessary to form the second assembly in the form of the first assembly and then turn .it over, as it would usually be more convenient to merely turn the stacks of leg and yoke laminations over before assembling the second basic assembly. I

More specifically, the second basic assembly 110 includes first and second groups 74' and 76' of leg laminations, and first and second groups 78 and 80 of yoke laminations. The groups located in the same relative positions as those of the first basic assembly 90 are given the same reference numeral as those of the first assembly, with the prime marks added to distinguish them. The second basic assembly 110 defines an opening or window 29", which has corners 92", 94", 96", and 98", and the outer configuration of the second basic assembly 110 has corners 100", 102", 104", and 106". The void volumes generated at window corners 92", 94", 98" and 96", are in the yoke, leg, yoke and leg portions, respectively, opposite to the locations of the void volume in like corners of the first basic assembly.

The next step is superposing the second basic assembly 110 on the first basic assembly 90, with their openings29" and 29 aligned. This provides a composite assembly of first and second basic assemblies, and forms the basic core structure 9 shown in FIG. 7. The magnetic core 9 shownin FIG. 7, however, may have as many superposed composite assemblies as required to obtain the des'iredbuild dimensionof the magnetic core. k

The step of disposing the second assembly 110 on the first assembly 90 places the layers of the twogroups together which have the longest laminations therein. Thus, the shortest laminations are in the bottom and top layers of the composite group, and the longest laminations are at the midpoint of the group. This placement and orientationof the first and second basic assemblies 90 and 110, respectively, provides a continuous, stepped-lap joint at each comer of the magnetic core, which has almost twice the number of steps per basic pattern as'the number of laminations in each basic assembly of groups. Further, the step pattern progre sses around each corner of the magnetic core.

FIG. 8 is a cross sectional view of the stepped-lap joint located between the inner corner 96 and the outer corner 104 of the magnetic core 9 shown in FIG. 7, with the cross sectional view being taken in the direction of arrows VIIIVIII. It will be noted that the joint 119 located in the bottom layer of the composite assembly starts on the left-hand side of the corner, with the corner being represented by the line 120, and then the stepped joint moves incrementally towards the c'orher, from layer to layer, until reaching the last lamination of the first assembly, with this joint being aligned with the corner. The stack height of the first assembly is indicated by the dimension 124. The second basic assembly, represented by the stack dimension 122, has the joint of its bottom layer aligned with the corner 120, and thus it is also aligned with the joint in the top layer of the first basic assembly. The joints then continue on the right-hand side of corner 120, incrementally stepping through the layers of laminations until reaching the top layer of the second basic assembly. Therefore, a single substantially continuous steppedlap pattern having nine steps therein has been created with [0 layers of laminations.

It is important to note that since the void volumes at the window corners of the first and second assemblies are located in the yoke portion in one assembly and in the leg portion in the other assembly, that the effective length of the flux path has been reduced, as the void I 8 volume appears to be located in both the leg and yoke por'tionsfbut with each being about one-half the void volume of eitherbasic group appearing alone. The effective void volume'at the outer corners of the magnetic core 9 is also reduced, as the clipped ends of the longest laminations extend completely to the geometrical corners of the magnetic core, with'the alternating locations of the first and second basic groups at the outer corners of the core alternately filling different areas of the corner with magnetic material, resulting in a smaller effectivevoid volume due to'this averaging effect.

It will be noted that while the basic joint pattern appears on both sides of the corners of the magnetic core 9, that this does not complicate stacking, nor does it make the developmentof the stepped pattern by aligning incrementally clipped ends impractical, due to the stacking and orientation of the laminations in groups,

and turning the groups over to provide the continuation of the stepped pattern. i

While the example disclosed in setting forth the steps of the new and improved method of constructing a stepped-lap magnetic core illustrates two joints being aligned at the corners of the magnetic core, it is to be understood that the alignment of two adjacent joints may be precluded by eliminating the top and bottom layers of the first'and second basic assemblies, respectively. In this type of arrangement, there wouldbe a double increment between the steps at the-comers of the composite assembly, but the number of steps in the basic pattern would be equal to the number of layers in the composite assembly. Further, this arrangement would provide a small void in the layer containing the longest laminations, at the corners of the opening or window, and the unclipped ends of the longestlamination would not extend to the geometrical outer comers of the magnetic core. I 7 2 Three single-phase shell-form magnetic "cores for transformers rated 24 M.V.A., were constructed in the teachings of the prior art. Core No. l was constructed with butt-lap joints as disclosed in US. Pat. No. 2,300,964, Core No. 2 was constructed with stepped-lap joints having seven steps and a A inch lap, with the seven step pattern being located on only one side of each corner of the magnetic core, such as disclosed in US. Pat. No. 3,l53,2l5 and Core No. 3 was constructed according to the teachings of the present invention, flipping alternate groups of laminations, with each group having a seven step pattern to provide a continuous fourteen step pattern which progressed in a continuous manner from one side of the corner of the magnetic core to the other. The core losses (TW) and exciting volt-amperes (AW) for each of these cores is listed in table I.

v TABLE 1 Core No. l Core No. 2 Core No. 3

TW (Watts) 31.979 31,073 29,006 176.800 68.640 45.670

It will be noted from table l,,that Core No. 3, which was manufactured according to the teachings of themvention, has a very decidedv advantage over the other amperes.

In summary, there has been disclosed a new and improved method of constructing a magnetic core with stepped-lap joints, which method may utilize a shear to cut the laminations from a single strip of magnetic, metallic material, as all of the cuts are straight cuts made completely across the width dimension of the strip of magnetic material. The method provides substantially twice the number of steps in the basic group pattern, without doubling the number of different lamination dimensions, and the basic pattern includes joints on both sides of the corners of the magnetic core while retaining the advantage of being able to incrementallyspace the laminations with incremental clips. Further,

the incremental clipping of the laminations is required on only one end of each lamination, enabling the clipping step to be performed while the lamination is being sheared, and the magnetic core resulting from the new and improved method has no protrusions extending beyond the sides of the magnetic core, facilitating the use of the magnetic core with a form-fit type of tank.

While the invention has been described relative to magnetic cores which have a substantially rectangular cross-sectional configuration, it is to be understood that it may also be applied to magnetic cores having a cruciform cross-sectional configuration. In this instance, the invention would apply to each width of lamination used in the cruciform construction.

We claim as our invention: 1. A method of constructing a magnetic core of the stacked type, comprising the steps of:

providing a strip of metallic, magnetic material having a predetermined width dimension, diagonally cutting the strip across its complete width dimension at predetermined spaced locations to provide a plurality of substantially trapezoidal shaped leg laminations having like length dimensions and a plurality of substantially trapezoidal shaped yoke laminations having like length dimensions, incrementally clipping only one end of the leg laminations to provide a predetermined number of leg laminations having different maximum length dimensions, incrementally clipping only one end of the yoke laminations to provide a predetermined number of yoke laminations having different maximum length dimensions, stacking the leg laminations to provide a plurality of similar groups, with the clipped ends ofeach group being against a common plane to offset the diagonally cut ends of each group in a stepped-lap relationship, and with each group including at least one of each of the different lengths of leglaminations, stacking the yoke laminations to provide a plurality of similar groups, with the clipped ends of each group being against a common plane to offset the diagonally cut ends of each group in a stepped-lap relationship, and with each group including at least one of each of the different lengths of yoke laminations, assembling two of the stacked groups of leg laminations and two of the stacked groups of yoke laminations to provide a first substantially rectangular assembly having an opening therein, with the diagonally cut ends of the leg and yoke laminations butt- 10 ing to provide diagonal corner joints having a stepped-lap pattern on a predetermined side of each corner of the first assembly, and with the clipped ends of each group adjacent unclipped ends of another group,

providing a second assembly of groups of leg and yoke laminations which is similar to the first, except in rotational symmetry therewith, about an axis of the first assembly which is parallel with the major planes of the laminations therein, to place the stepped-lap joints on the side of each corner which is opposite to the predetermined side on which they are located in the first assembly,

and superposing the second assembly on the first assembly with their openings aligned, to provide a composite assembly having stepped-lap joints on each side of each outer corner thereof.

2. The method of claim 1 wherein the steps of assembling the groups of leg and yoke laminations to provide the first and second assemblies, and superposing the first and second assemblies, orients the stepped-lap joints at the comers of the assemblies to provide a single, substantially continuous, stepped-lap pattern between opposite sides of the composite assembly at each outer corner, crossing each outer corner at substantially the midpoint of the pattern.

3. The method of claim 1 wherein the step of superposing the second assembly on the first assembly includes orienting the assembly such that void volume appearing at each comer of the substantially rectangular opening of the composite assembly, is in the yoke portion in one assembly and in the leg portion of the other assembly, to reduce the effective void volume at these corners of the composite assembly.

4. The method of claim 1, wherein the steps of assembling the groups of leg and yoke laminations places the tip of the unclipped diagonally cut end of the longest lamination of each group substantially at an outer geometrical comer of the composite assembly.

5. The method of claim 4, wherein the steps of assembling the groups of leg and yoke laminations to provide the first and second assemblies, and superposing the first and second assemblies to provide the composite assembly, reduces the effective void volume at the outer corners of the composite assembly by placing the unclipped ends of the groups of leg and yoke laminations in the yoke in one group, and in the leg in the next group, for each outer corner of the composite assembly.

6. The method of claim 1, wherein the steps of stacking the leg and yoke laminations into groups directs the longest lamination of each group for the first assembly to the first layer of the first assembly, and the shortest lamination of each group to the last layer of the first assembly, and the longest lamination of each group for the second assembly to the last layer of the second assembly, and the shortest lamination to the first layer, with the step of superposing the second assembly on the first assembly, placing the last layer of the second assembly on the first layer of the first assembly.

7. The method of claim 1 wherein the step of assembling the groups of leg and yoke laminations assembles the layer having the longest laminations with substantially no void volume at the inner corners of the assembly. 

1. A method of constructing a magnetic core of the stacked type, comprising the steps of: providing a strip of metallic, magnetic material having a predetermined width dimension, diagonally cutting the strip across its complete width dimension at predetermined spaced locations to provide a plurality of substantially trapezoidal shaped leg laminations having like length dimensions and a plurality of substantially trapezoidal shaped yoke laminations having like length dimensions, incrementally clipping only one end of the leg laminations to provide a predetermined number of leg laminations having different maximum length dimensions, incrementally clipping only one end of the yoke laminations to provide a predetermined number of yoke laminations having different maximum length dimensions, stacking the leg laminations to provide a plurality of similar groups, with the clipped ends of each group being against a common plane to offset the diagonally cut ends of each group in a stepped-lap relationship, and with each group including at least one of each of the different lengths of leg laminations, stacking the yoke laminations to provide a plurality of similar groups, with the clipped ends of each group being against a common plane to offset the diagonally cut ends of each group in a stepped-lap relationship, and with each group including at least one of each of the different lengths of yoke laminations, assembling two of the stacked groups of Leg laminations and two of the stacked groups of yoke laminations to provide a first substantially rectangular assembly having an opening therein, with the diagonally cut ends of the leg and yoke laminations butting to provide diagonal corner joints having a stepped-lap pattern on a predetermined side of each corner of the first assembly, and with the clipped ends of each group adjacent unclipped ends of another group, providing a second assembly of groups of leg and yoke laminations which is similar to the first, except in 180* rotational symmetry therewith, about an axis of the first assembly which is parallel with the major planes of the laminations therein, to place the stepped-lap joints on the side of each corner which is opposite to the predetermined side on which they are located in the first assembly, and superposing the second assembly on the first assembly with their openings aligned, to provide a composite assembly having stepped-lap joints on each side of each outer corner thereof.
 2. The method of claim 1 wherein the steps of assembling the groups of leg and yoke laminations to provide the first and second assemblies, and superposing the first and second assemblies, orients the stepped-lap joints at the corners of the assemblies to provide a single, substantially continuous, stepped-lap pattern between opposite sides of the composite assembly at each outer corner, crossing each outer corner at substantially the midpoint of the pattern.
 3. The method of claim 1 wherein the step of superposing the second assembly on the first assembly includes orienting the assembly such that void volume appearing at each corner of the substantially rectangular opening of the composite assembly, is in the yoke portion in one assembly and in the leg portion of the other assembly, to reduce the effective void volume at these corners of the composite assembly.
 4. The method of claim 1, wherein the steps of assembling the groups of leg and yoke laminations places the tip of the unclipped diagonally cut end of the longest lamination of each group substantially at an outer geometrical corner of the composite assembly.
 5. The method of claim 4, wherein the steps of assembling the groups of leg and yoke laminations to provide the first and second assemblies, and superposing the first and second assemblies to provide the composite assembly, reduces the effective void volume at the outer corners of the composite assembly by placing the unclipped ends of the groups of leg and yoke laminations in the yoke in one group, and in the leg in the next group, for each outer corner of the composite assembly.
 6. The method of claim 1, wherein the steps of stacking the leg and yoke laminations into groups directs the longest lamination of each group for the first assembly to the first layer of the first assembly, and the shortest lamination of each group to the last layer of the first assembly, and the longest lamination of each group for the second assembly to the last layer of the second assembly, and the shortest lamination to the first layer, with the step of superposing the second assembly on the first assembly, placing the last layer of the second assembly on the first layer of the first assembly.
 7. The method of claim 1 wherein the step of assembling the groups of leg and yoke laminations assembles the layer having the longest laminations with substantially no void volume at the inner corners of the assembly. 